Total knee replacement (TKR) is a common surgical procedure that replaces the worn surfaces of the thigh bone (femur) and shin bone (tibia) with artificial components. Its primary goals are to alleviate chronic knee pain, restore function, and improve mobility for individuals with severely damaged knee joints.
The Role of ACL and PCL in Knee Function
The knee joint relies on strong ligaments for stability and controlled movement, including the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). The ACL is located in the front of the knee, preventing the shin bone from sliding too far forward relative to the thigh bone. It also helps to limit excessive rotation and hyperextension.
Conversely, the PCL is situated behind the ACL. It functions as the main restraint against the shin bone moving too far backward on the thigh bone. The PCL also guides the natural rolling back (femoral rollback) of the thigh bone during knee bending, which is essential for efficient knee mechanics and overall stability.
What Happens to the ACL During Total Knee Replacement
During a conventional total knee replacement, the anterior cruciate ligament (ACL) is nearly always removed. This is primarily due to the design of most prosthetic components and the surgical cuts required to prepare the bone. The artificial joint surfaces are engineered to provide inherent stability, effectively taking over the ACL’s role in controlling anterior motion of the shin bone.
The decision to remove the ACL also stems from the fact that in many severely arthritic knees, the ligament is already damaged or degenerated. Even if the ACL is present, its function may be compromised, and retaining it could lead to unpredictable kinematics or long-term issues. While some newer implant designs are exploring ACL preservation, the vast majority of successful total knee replacements have involved its removal.
What Happens to the PCL During Total Knee Replacement
The posterior cruciate ligament (PCL) can either be retained or removed during total knee replacement, unlike the ACL. This decision depends on several factors, including the PCL’s condition, the severity of knee deformity, and the surgeon’s preference. When the PCL is healthy and well-balanced, retaining it can offer potential advantages by preserving some of the knee’s natural mechanics.
Retaining the PCL allows it to continue guiding femoral rollback, which can contribute to a more natural feel and potentially improve quadriceps efficiency. This preservation may also help maintain proprioception. However, if the PCL is contracted, damaged, or cannot be properly balanced during surgery, its removal may be necessary to achieve optimal joint alignment and stability. Sacrificing the PCL can simplify the surgical procedure and allow for more predictable joint balancing.
Implant Designs and Ligament Management
The decision to retain or sacrifice the PCL directly influences the choice of prosthetic components used in total knee replacement. Two main categories of knee implants are available: PCL-retaining (Cruciate-Retaining or CR) designs and PCL-substituting (Posterior-Stabilized or PS) designs. Each design is engineered to function optimally with either the presence or absence of the PCL.
PCL-retaining implants work in conjunction with a preserved PCL, relying on its natural function to provide posterior stability and guide thigh bone motion during bending. These implants typically have a relatively flat tibial insert, allowing the retained PCL to control kinematics. In contrast, when the PCL is removed, PCL-substituting implants are used. These incorporate a specialized cam-and-post mechanism that replicates the PCL’s function by preventing backward shin bone sliding and promoting femoral rollback. The selection between these implant types is a complex decision, often based on the patient’s specific knee anatomy, bone quality, and the surgeon’s clinical judgment.